1. Field of Invention
The present invention relates to an ultrasonic detecting method suitable for a bearing ring of a tapered roller type rolling bearing or a cylindrical roller type rolling bearing such as a roll neck bearing for steel, in particular, to which a large load is applied.
2. Related Art
Conventionally, as an ultrasonic detecting method for detecting a defect within a bearing ring such as an inner race or an outer race, there has been known a normal bean method in which, during the manufacturing process of a steel product for a bearing ring, the steel product having been rolled is placed within the water or on a stand and an ultrasonic wave is transmitted from the outer peripheral surface to the inner portion of the steel product thereby to detect the defect of the bearing ring (see Special Steel, vol. 46, No. 6, page 31, edited by Special Steel Club Co.).
Further, as an ultrasonic detecting method for detecting a defect within or on the surface of a rolling element such as a ball, roller or the like, there has been employed in many cases a method in which an ultrasonic wave with a frequency in a rage of 15 MHz to 50 MHz is transmitted to the rolling element to detect a microscopic defect.
However, the aforesaid conventional ultrasonic detecting method for a steel product for a bearing ring mainly has been developed so as to detect a blow hole within the steel product or a defect such as a non-pressed portion or the like of the steel product in the rolling process, but not developed so as to detect a microscopic defect in the vicinity of the surface of the steel product or a large non-metallic intervening material of about several 100 .mu.m within the steel product.
This is because the dead zone just beneath the surface of the steel product at the time of performing the ultrasonic detection is large since the surface of the steel product having been subjected to the rolling process and left as it is is rough. Further, since there is a bend, torsion or the like in the steel product, it is difficult to maintain the distance between the steel product and the probe constant, and in particular it is impossible to detect a microscopic defect in the vicinity of the surface of the steel product Furthermore, in the case of detecting a defect within the steel product with such a large diameter exceeding 100 mm, since it is required to set the detecting frequency to a low frequency in order to prevent the degradation of the sensitivity due to the attenuation of the ultrasonic wave, only a defect of about several mm can be detected.
The ultrasonic detecting method used for inspecting a rolling element mainly has been developed so as to efficiently detect a flaw or a microscopic surface defect of the rolling element. Such an object of this ultrasonic detecting method will be understood from a fact that a very high frequency in a rage of 15 MHz to 50 MHz capable of detecting a microscopic defect has been used in this method.
According to the recent technical development, it has become possible to detect a microscopic non-metallic intervening material of about 0.01 mm (10 .mu.m), for example, by using the ultrasonic wave of a high frequency (e.g., 50 MHz to 150 MHz). However, if the frequency is made higher, the attenuation degree of the ultrasonic wave within the steel product becomes larger (the attenuation degree of the ultrasonic wave becomes further larger if the degree of the roughness of the surface of the steel product becomes larger), so that the defect detection can be performed only in the range from the surface of the steel product to a position beneath by about 3 mm from the surface, for example,. Accordingly, it has been impossible to efficiently inspect a product such as a bearing ring, in particular, which is required to be inspected as to a defect at the inner portion thereof.
The total amount of non-metallic intervening material contained in the steel product used for a bearing ring has been reduced due to the recent improvement of the steel manufacturing technique. In particular, the generation frequency of a defect such as a macro-streak-flaw or the like which is a typical example of a large non-metallic intervening material has been reduced. However, such a defect as a macro-streak-flaw or the like sometimes remains within a steel product, and hence there has been a case that the premature breakaway of the bearing occurs due to the macro-streak-flaw or the like. Accordingly, it has been desired to provide a method for effectively detecting in advance a defect such as a macro-streak-flaw or the like remained within a steel product in order to improve the reliability of a bearing.
For example, since a bearing such as a steel rolling bearing or the like is applied with a very large load, the maximum shearing stress appears within a steel product in a range from the position near the surface thereof to such a deep position of several mm from the surface thereof. Accordingly, it is required to detect not only a defect just beneath the surface of the steel product but also a defect at the deep position within the steel product.
An example of a method for detecting the distribution of relatively small non-metallic intervening material within a steel material by increasing the ultrasonic wave frequency is disclosed in Unexamined Japanese Patent Unexamined Publication Hei. 9-257761. However, in this method, since the detection is performed after adjusting the surface roughness of a sample of the steel product by the grinding finishing process, it is difficult to detect all the steel products.